A typical transparent liquid crystal display (LCD) includes a layer of liquid crystal material sandwiched between front and back transparent plates. The back plate includes transparent electrodes forming pixels on a transparent substrate such as glass. The front plate includes a transparent ITO electrode forming a common plate. The front and back transparent electrodes are coated with polyimide alignment layers. In active matrix displays, there is a silicon active backplane layer with electronic pixel addressing circuits on top of the transparent substrate of the back plate, and the transparent pixel electrodes are on top of and controlled by this silicon layer. In order for a liquid crystal display to function properly, the light incident on the liquid crystal material must be polarized. Thus, polarizers are typically used with LCD displays.
The applicants hereof have noted the advantages of using sapphire as the substrate for the silicon active backplane. Notably, sapphire is transparent and promotes the growth of single crystal silicon in which the electronic circuits are formed. See U.S. Pat. Nos. 6,190,933; 6,312,968; 6,365,936; 6,521,950, and 6,617,187, all of which are incorporated herein by this reference. Unfortunately, sapphire de-polarizes the light that also passes through the liquid crystal material.
The active matrix pixel addressing circuits can also depolarize the light, in which case the liquid crystal is prevented from controlling the light to obtain a high on/off contrast ratio. As this electrode circuitry is made smaller and smaller, as is the case with microdisplays, the depolarization effect is even greater.
A microdisplay is typically a tiny display measuring less than 1.5″ diagonally that can be used with lenses to display the image of a full-size computer monitor or television. Microdisplays are used to create the image in such products as conference room projectors, rear-screen large format televisions, and camcorder viewfinders. Most microdisplays have electronic pixel addressing circuits fabricated as integrated circuits in a silicon layer. The active silicon layer is generally on a round substrate wafer that is typically made of silicon, glass, quartz, or sapphire. A number of complete microdisplay active backplanes are repeated over the surface of the wafer, which is ultimately cut up to make individual microdisplays.
Furthermore, the undesired depolarization effects of the transparent substrate and pixel addressing circuits tend to depend on the illumination incident angle, and this depolarization can be severe at certain angles. A microdisplay intrinsically has a large cone angle of light because the periodic small scale active matrix backplane structure diffracts light into a cone of off-axis diffraction order directions. In addition, obtaining high optical system throughput in microdisplay applications such as a projectors generally requires low f/# optical systems that transmit light having a large cone angle of transit directions through the microdisplay.
It is known to coat a polarizing material over one or both of the inner surfaces of a liquid crystal cell in order to protect the polarizers from mechanical damage and to eliminate the additional manufacturing step of attaching polarizers to a display. See U.S. Pat. Nos. 3,941,901, 6,630,289, and 6,399,166. In some instances, the polarizing material may also serve as the liquid crystal alignment layer.
It is also known to incorporate a wire grid polarizer behind the layer of liquid crystal material for passing light of one polarization orientation through the wire grid polarizer and reflecting light of another polarization orientation back through the layer of liquid crystal material to allow the display to operate either with ambient illumination or internal illumination. See U.S. Pat. No. 5,986,730 incorporated herein by this reference. It is also known to incorporate a wire grid polarizer behind the layer of liquid crystal material to act as an electrode, polarizer, and a mirror. See U.S. Pat. No. 4,688,897 also incorporated herein by this reference.
No prior art, however, teaches or suggests a technique for improving the contrast of a microdisplay incorporating a sapphire substrate.